As is commonly known a mobile communication network such as a GSM-system is hierarchically structured as follows. A mobile services switching center MSC controls at least one base station controller BSC. One base station controller in turn controls at least one base transceiver station BTS which covers a designated communication area. Said communication area or coverage is called a cell. The base transceiver station communicates with a respective mobile station MS within this cell.
When the mobile station moves within the communication network it has to be ensured that the connection between the mobile station and the communication network is not interrupted even when the mobile station leaves the cell (i.e. the coverage area) of one base transceiver station and enters another cell. The procedure used for maintaining connection is referred to as a handover.
During such a handover procedure the connection of the mobile station to the communication network is changed from one cell to another cell. Additionally, this base transceiver station handover can be combined with a base station controller handover when the respective base transceiver stations are controlled by different base station controllers. Moreover, a handover can also occur between two mobile services switching centers in a corresponding case. Furthermore, a handover is also possible between communication networks using different technologies such as a handover between a GSM network and a wideband code division multiple access (WCDMA) network or between a GSM network and a radio local area network (RLAN).
To decide whether a handover is necessary, certain measurements are required. By these measurements, e.g., a signal quality, such as quality of the connection, signal strength, bit error rate and the like, is checked for several monitored base transceiver stations. The corresponding results are used to decide whether a handover is required. For example, the signal strength of the current base transceiver station decreases while the signal strength of another base transceiver station increases. If a signal strength ratio reaches a predetermined level (i.e., the signal quality of the currently used base transceiver station is lower than a predetermined threshold level), a handover from the currently used base transceiver station to the other base transceiver station having a better signal quality will be initiated.
To inform the mobile station as to which base transceiver stations are available (i.e. in which coverage areas the mobile station is) and to make necessary information for said measurement available, the base transceiver stations use signaling channels to send and receive this information. One of said signaling channels is a broadcast control channel (BCCH) which is permanently broadcasted by a respective base transceiver station, e.g. in a certain timeslot of a TDMA frame. Said BCCH can be compared with a radio beacon of the base transceiver station, wherein via said BCCH the first contact between the mobile station and the base transceiver station is established. The BCCH is also used to inform the mobile station about access parameters and the like and to enable a position determination.
On the other hand, a handover can also be required in the case, that there is to much traffic in the corresponding cell. Each cell has a limited number of traffic channels (TCH) used for data exchange or speech transmission. If a subscriber wants to make a call but no more TCH is available because of to many currently active calls, a handover is performed to another cell with an available TCH, even if measurement of, e.g., signal strength results in a lower value for this new cell.
It is to be noted, that a handover procedure can be o executed during a call setup phase and during an active call.
However, as only a limited number of channels is available, a permanent transmission of a channel like said BCCH is wasteful in terms of the radio communication resources of the communication network.
Additionally, each of the base transceiver stations of the communication network has permanently to be turned on to transmit said BCCH, even if there is no mobile station present within its coverage area. This results in an unnecessary power waste.
Document U.S. Pat. No. 5,546,445 describes a method and a system in which both a position controlled system and a signal strength system are included in a cellular system, a mobile telecommunications switching office can include a software system in a memory to use the position controlled system, but to also test signal strength, and to use a signal strength controlled system if a signal still falls below a predetermined value when making call management decisions based on the position of a mobile unit.
Document WO 99/07167 concerns a geographical restriction in a cellular telecommunication network. There is disclosed a o method for performing a handover in which tariff related information are taken into consideration.
Document WO 93/19560 describes a method for performing a handover in which a serving base transceiver stations seeks base transceiver stations for a handover which are not adjacent to the serving base transceiver station.
Document DE 197 41 701 A1 describes a method and a device for a handover of a mobile station in a mobile radio o system. A base station is connected to a radio positioning system and receives positioning signals. Coordinates of a mobile station are calculated on the basis of the coordinates of the base stations communicating with the mobile station and a distance between the mobile station and the base stations.